1. Field of the Invention
This invention relates to a method of and apparatus for controlling a velocity of an industrial robot such as a welding robot which makes a playback operation between taught points.
2. Description of the Prior Art
Referring to FIG. 6, there is shown in a diagrammatic representation an exemplary welding process which is taken by an industrial robot such as, for example, a welding robot. In a section between points P2 and P3 or between points P5 and P6 indicated by notched lines in FIG. 6, arc welding is performed at a velocity within an allowable velocity range which is determined in accordance with a designated welding leg length in advance. To the contrary, in any one of the other sections of P1-P2, P3-P4-P5 and P6-P7, the robot is moved in order to move to the welding section P2-P3 or P5-P6, and each of the sections will be hereinafter referred as to an air cut section. In any air cut section, there is no limitation in velocity of the robot from a welding operation, and actually, the velocity of the robot here is determined in accordance with performances of motors of individual axes and so forth of the robot, that is, in accordance with the performance of the robot itself.
Where the velocity of movement of an end of an arm of the robot in any air cut section is set, normally the velocity of the robot is controlled in accordance with a velocity pattern as indicated by a chain line in FIG. 7. As seen from the velocity pattern, predetermined periods of time are required for an acceleration section to an instruction velocity V.sub.0 and for a deceleration section to stopping or zero velocity.
Meanwhile, an allowable velocity Vmax is set in advance. Such allowable velocity Vmax is the maximum velocity of the robot, and in the case of a lower arm of an articulated robot, the allowable velocity Vmax is limited by a maximum velocity of rotation of a driving motor for a lower arm axis S2 and is about 80 m/min or so. Then, if the instruction velocity V.sub.0 is higher than the allowable velocity Vmax, this will lead to an error in operation of the robot. Accordingly, at a point of time at which the allowable velocity Vmax is reached during acceleration of the robot as indicated by a solid line in FIG. 7, the control cycle is elongated in accordance with the instruction velocity V.sub.0 and the allowable velocity Vmax, for example, to (V.sub.0 /Vmax).times.(ordinary control cycle) so that the velocity of the robot may not be accelerated to the instruction velocity V.sub.0 and maintain the allowable velocity Vmax. Then, the velocity of the robot is decelerated from the allowable velocity Vmax. In particular, the velocity of the robot is controlled in a substantially trapezoidal velocity pattern which is obtained by cutting an upper portion of the ordinary velocity pattern shown by a chain line in FIG. 7 away along a horizontal line of the allowable velocity Vmax. Such velocity control is disclosed, for example, in Japanese Patent Laid-Open No. 63-80307.
With the velocity control, after an instruction velocity V.sub.0 is taught, there is no necessity of checking, in a test mode, whether or not the instruction velocity V.sub.0 is higher than the allowable velocity Vmax and an error may take place. Accordingly, teaching can be performed rapidly.
However, with such a conventional velocity controlling means as described above, since a function of moderated acceleration and deceleration is not taken into consideration, in case the instruction velocity V.sub.0 is higher than the allowable velocity Vmax, the robot is controlled in such a velocity pattern as indicated by a solid line in FIG. 7. Consequently, two edges E are produced at a point of time at which the robot moves from the acceleration section to the section of the allowable velocity Vmax and another point of time at which the robot moves from the section of the allowable velocity Vmax to the deceleration section. At each of such edges E, high vibrations will be produced at an arm or some other components of the robot and have a bad influence on the accuracy in control of the robot.
Further, with the conventional controlling means described hereinabove, an actual velocity of the robot is sometimes different from an instruction velocity. In such an instance, however, there is no measures for an operator to find out an actual velocity of movement of the robot. Accordingly, it is expected to enable an industrial robot to inform an operator or the like of an actual velocity of the robot.